1. Field of the Invention
The present invention relates to a photoelectric conversion device (transducer) such as an image sensor used, for example, as an image reading means in hand-held scanners and facsimile machines.
2. Description of the Prior Art
Image sensors come in various types. The most typical prior art image sensor is one in which numerous pairs of photodiodes and blocking diodes arranged linearly in a primary scanning direction are divided into as many blocks as the number of input circuit channels, the blocks being connected into a linear pattern.
The above prior art image sensor works as follows. The photodiodes accumulate electrical charges upon receipt of the light reflect from an original document. When one input circuit supplies a pulse to the blocking diodes in the corresponding block, these diodes are turned on, causing the charges in the photodiodes of the same block to flow into the output circuits through as many lead wires as there are output circuit channels. Each input circuit supplies a pulse to the corresponding block in turn for the sequential scanning of the blocks, thereby to read image data of one line.
Each paired photodiode and blocking diode is formed on a glass substrate of amorphous silicon (.alpha.-Si) or like semiconductor material in a so-called pin junction containing successive "p", "i" and "n" layers from the bottom up, a shared lower electrode or contact connecting the anodes of both diodes. The top surface of the semiconductor layers of the photodiode has a transparent conductive film that allows the reflected light from the document to pass through. The entire diode assembly would, of course, be covered with an insulation film with the exception of part of the transparent conductive film or layer and of the lead wire for electrical connection of the diodes to each other. Respective upper electrodes or contacts are formed on top of the paired photodiode and blocking diode, and connect the photodiode to a lead wire and the blocking diode to the input circuit. A protective film is provided as needed over the diode assembly.
U.S. Pat. No. 4,369,372 discloses an image sensor having photodiodes and blocking diodes formed on the common lower electrodes, as outlined above. In crystalline silicon integrated circuits used for image sensor applications, the use of aluminum (Al) for electrodes and lead wires is generally preferred because of its relatively low electrical resistivity. Aluminum can, however, have a number of disadvantages. One disadvantage of aluminum is its weak adhesiveness to the glass substrate. For example, the adhesiveness of aluminum to the glass substrate is only one seventh that of chromium (Cr).
Furthermore, the lower electrodes of blocking diodes and photodiodes must be made of a thermally stable metal that will not mix with the semiconductor at temperatures as high as 250.degree. C. during production of the semiconductor layers. At such high temperatures, aluminum tends to diffuse into the semiconductor.
The lower electrodes are required to make an ohmic junction with the semiconductor layer formed immediately thereabove. In this sense, too, aluminum is not a preferred substance for forming the lower electrodes.
With the above-described properties taken into account, the lower electrodes of blocking diodes and photodiodes are usually made of such metals as chromium (Cr), molybdenum (Mo), tantalum (Ta) and MoTa, which have high melting points, high adhesiveness and thermal stability.
Since lead wires are directly formed on the glass substrate, they may be produced during the same process as the lower electrodes. Thus, the lead wires may be made of the same metal as the lower electrode, such as chromium, the selection being made in consideration of the metal's adhesiveness to the glass substrate and the efficiency of its use in the production process. On the other hand, the upper electrode, not facing the same constraints as the lower electrodes and lead wires, may advantageously be formed of aluminum. In this manner, lower electrodes and lead wires of the prior art diode assembly are formed from chromium or other metals of high melting point, while the upper electrode thereof are formed from aluminum.
Whereas the use of high-melting-point metals for the lower electrodes and the lead wires has advantages as described above, these metals have a disadvantage of a higher electrical resistivity than that of aluminum. For example, while the electrical resistivity of aluminum is 2.5.times.10.sup.-8 .OMEGA.m at 0.degree. C. and 3.55.times.10.sup.-8 .OMEGA.m at 100.degree. C., the electrical resistance of chromium (resistance for a volume of unit area and unit length) is 12.7.times.10.sup.-8 .OMEGA.m at 0.degree. C. and 16.1.times.10.sup.-8 .OMEGA.m at 100.degree. C. If the wiring of an image sensor is formed from a metal of such high resistivity, the time constant of the wiring becomes greater. This results in a reduced response time of the image sensor, which will have its effect in the form of a lower reading speed. Because the peak value of the image signal drops with higher resistance, the sensitivity of the image sensor is lowered and its S/N ratio worsens correspondingly.
In particular, the lead wires play a large part in increasing the electrical resistance of the linear connection as a whole. This is because the lead wires for the primary scanning direction are very long. For example, while the lower electrodes for the secondary scanning direction are 1 mm long at most, the lead wires for the primary scanning direction are as long as 108 mm for an A6-size image sensor and 216 mm for an A4-size image sensor.
One way to reduce the electrical resistance of the lead wires is to increase their width. However, increasing the width of the lead wires proportionately widens the area of the crossover formed between the lead wires and the upper electrodes. A large crossover capacity of the crossing wires and electrodes promotes crosstalk between them. The correspondingly lowered MTF (modulation transfer function) deteriorates the image quality obtained. Thus, there is a limit to widening the lead wires for the purpose of reducing their electrical resistance.
Another way to lower the electrical resistance of the lead wires is to increase their thickness. Chromium wires are typically made thousands of angstroms thick by evaporation. However, if attempts were made to increase their thickness, the stress that would develop therein during growth would make it impossible to form a stable thin film thereof.
Even if a thicker chromium film were somehow obtained, the lower electrodes would be correspondingly thickened since they are formed during the same deposition step in which the lead wires are formed. This would add to the total thickness of the diode assembly when combined with the semiconductor layers. As a result, the insulation film formed over these components would tend to have pronounced curvature near the periphery of the photodiodes and blocking diodes, and cracks would likely appear there.